FIG. 1 shows an example of a circulating fluidized bed furnace 3 comprising a combustion furnace 1 and a fluidized bed gasification furnace 2 for production of a produced gas. Prior art literature for such circulating fluidized bed furnace is, for example, Patent Literature 1.
In the combustion furnace 1 constituting the circulating fluidized bed furnace 3 of FIG. 1, air or oxygen 4 is supplied from below, and at least either startup fuel 5 or char referred to hereinafter is supplied to a lower side portion of the furnace to bring about fluidized combustion and thus heating of circulating particles. Combustion fluid 6 from the combustion furnace 1 is guided to a separator 9 where it is separated into exhaust gas 7 and circulating particles 8.
Guided into the fluidized bed gasification furnace 2 (gas production furnace) constituting the circulating fluidized bed furnace 3 and filled with the bed material such as sand on a diffuser 11 are the separated hot circulating particles 8 from the separator 9 as well as a raw material 10 such as coal or biomass from above. A portion of the furnace below the diffuser 11 is supplied with steam S as gasifying agent from an auxiliary steam device 12 such as an auxiliary boiler or other facility to thereby form a fluidized bed 13 (bed) for gasification (steam gasification) of the raw material 10, and a produced gas 14 which is a mixture of gas components such as hydrogen (H2), carbon monoxide (CO) and methane (CH4) produced at a freeboard section F above the fluidized bed (bed) is lead out through a top of the furnace. A mixture 17 of the circulating particles 8 with unreacted char in the gasification furnace 2 is returned through a lead-in pipe 18 to the combustion furnace 1 where the char is burned to heat the circulating particles 8. The gasification furnace 2 is provided with a mechanism for blocking of gas movement between the furnaces 1 and 2.
The produced gas 14 produced in the gasification furnace 2 is taken out through a lead-out pipe 15 and is fed to an objective device 16 through a solid separator, a gas refiner and the like (not shown).
The exhaust gas 7 separated from the circulating particles 8 in the separator 9 is guided by an exhaust gas pipe 19 to an induced draft fan 20 and is discharged through a stack 21.
With the circulating fluidized bed furnace 3 being shutdown, the fluidized bed gasification furnace 2 is generally filled with air. Thus, when the circulating fluidized bed furnace 3 is to be started up in this state, there is a fear that the combustive produced gas 14 in the gasification furnace 2 may be mixed with the air to bring about fast and furious combustion. In order to overcome this, an interior of the gasification furnace 2 requires to be purged with inert gas before the startup of the circulating fluidized bed furnace 3.
When the circulating fluidized bed furnace 3 in operation is to be shut down, there is also a fear that admixture of air into the gasification facility filled with combustive produced gas may bring about fast and furious combustion. Thus, the interior of the gasification furnace 2 requires to be purged with inert gas also for shutdown of the gasification facility.
Conventionally, the fluidized bed gasification furnace 2 of the circulating fluidized bed furnace 3 as shown in FIG. 1 is purged such that the gasification furnace 2 is supplied, from below, with inert gas 23 such as nitrogen (N2) or argon (Ar) from an inert gas supply source 22.
In such purging of the gasification furnace 2, not only a produced gas produced from the raw material charged into the fluidized bed 13 of fluidized bed material in the gasification furnace 2 but also the produced gas at and downstream of the freeboard section F must be purged, so that inert gas 23 (N2) is generally required in an amount about five times as much as a capacity of the fluidized bed gasification furnace 2. Such requirement of inert gas 23 in large quantity brings about a problem that a large-sized inert gas producer is required, resulting in substantial increase in cost for purging.
Patent Literature 2 discloses an operating method for a partial gasification and decomposition apparatus wherein gas in the apparatus is purged with steam upon startup and shutdown of the apparatus.    [Patent Literature 1] JP 2005-04195A    [Patent Literature 2] JP 388171B